I. Introduction
This invention relates to electroplating nonconductors, and more particularly, to a process for electroplating the surface of a nonconductor using a conductive particulate coating containing a reducing agent as a base for direct electroplating. This invention is especially useful for the manufacture of printed circuit boards.
II. Description of the Prior Art
Nonconductor surfaces are conventionally metallized by a sequence of steps comprising catalysis of the surface of the nonconductor to render the same catalytic to electroless metal deposition followed by contact of the catalyzed surface with an electroless plating solution that deposits metal over the catalyzed surface in the absence of an external source of electricity. Metal plating continues for a time sufficient to form a metal deposit of the desired thickness. Following electroless metal deposition, the electroless metal deposit is optionally enhanced by electrodeposition of metal over the electroless metal coating to a desired thickness. Electrolytic deposition is possible because the electroless metal deposit serves as a conductive coating that permits electroplating.
Catalyst compositions useful for electroless metal plating are known in the art and disclosed in numerous publications including U.S. Pat. No. 3,011,920, incorporated herein by reference. The catalyst of this patent consists of an aqueous suspension of a tin--noble catalytic metal colloid. Surfaces treated with such catalysts promote the generation of electrolessly formed metal deposits by the oxidation of a reducing agent in an electroless plating solution catalyzed by the catalytic colloid.
Electroless plating solutions are aqueous solutions containing both a dissolved metal and a reducing agent in solution. The presence of the dissolved metal and reducing agent together in solution results in plate out of the metal in contact with a catalytic metal tin catalyst. However, the presence of the dissolved metal and reducing agent together in solution may also result in solution instability and indiscriminate deposition of metal on the walls of containers for such plating solutions. This may necessitate interruption of the plating operation, removal of the plating solution from the tank and cleaning of tank walls and bottoms by means of an etching operation. Indiscriminate deposition may be avoided by careful control of the plating solution during use and by use of stabilizers in solution which inhibit indiscriminate deposition, but which also retard plating rate.
Attempts have been made in the past to avoid the use of art electroless plating solution by a direct plating process whereby a metal may be deposited directly over a treated nonconductive surface. One such process is disclosed in U.S. Pat. No. 3,099,608, incorporated herein by reference. The process disclosed in this patent involves treatment of the nonconductive surface with a tin-palladium colloid which forms an essentially nonconductive film of colloidal palladium particles over the nonconductive surface. This is the same tin-palladium colloid used as a plating catalyst for electroless metal deposition. For reasons not fully understood, it is possible to electroplate directly over the catalyzed surface of the nonconductor from an electroplating solution though deposition occurs by propagation and growth from a conductive surface. Therefore, deposition begins at the interface of a conductive surface and the catalyzed nonconductive surface. The deposit grows epitaxially along the catalyzed surface from this interface. For this reason, metal deposition onto the substrate using this process is slow. Moreover, deposit thickness is uneven with the thickest deposit occurring at the interface with the conductive surface and the thinnest deposit occurring at a point most remote from the interface.
An improvement in the process of U.S. Pat. No. 3,099,608 is described in U.K Pat. No. 2,123,036 B, incorporated herein by reference. In accordance with the process described in this patent, following catalysis, a surface is electroplated from an electroplating solution containing an additive that is said to inhibit deposition of metal on the metal surface formed by plating without inhibiting deposition on the metallic sites over the nonconductive surface. In this way, there is said to be preferential deposition over the metallic sites with a concomitant increase in the overall plating rate. In accordance with the patent, the metallic sites are preferably formed in the same manner as in the aforesaid U.S. Pat. No. 3,099,608--i.e., by immersion of the nonconductive surface in a solution of a tin-palladium colloid. The additive in the electroplating solution responsible for inhibiting deposition is described as one selected from a group of dyes, surfactants, chelating agents, brighteners, and leveling agents. Many of such materials are conventional additives for electroplating solutions.
There are limitations to the above process. Both the processes of the U.S. and U.K. patents for electroplating require conductive surfaces for initiation and propagation of the electroplated metal deposit. For this reason, the processes are limited in their application to metal plating solutions of nonconductive substrates in areas in close proximity to a conductive surface. In addition, in practice, it has been found that the surface provided with metallic sites is not robust and does not stand up to those chemical treatment compositions used prior to the step of electroplating. For this reason, when the process is used for the manufacture of printed circuit boards, void formation is a significant problem resulting in rejection of circuit boards manufactured by the process.
Improvements in processes for direct electroplating of nonconductors that overcome the deficiencies in the processes of U.S. Pat. No. 3,099,608 and in U.K. Pat. No. 2,123,036 are disclosed in U.S. Pat. Nos. 4,895,739; 4,919,768; 4,952,286; and 5,276,290, all incorporated herein by reference. In accordance with the processes of these patents, an electroless plating catalyst, such as that disclosed in the aforesaid U.K. patent, is treated with an aqueous solution of a chalcogen, such as a sulfur solution, to convert the catalyst surface to a chalcogenide surface. By conversion of the surface to the chalcogenide conversion coating, the coating formed is both more robust and more conductive and electroless plating catalyst does not desorb from the surface during metallization. Consequently, in accordance with the process of said patents, it is possible to form printed circuit boards using formulations that would otherwise attack the catalyst layer such as those solutions used in patterned plating processes.
The processes of the aforementioned patents provide a substantial improvement over the process of the U.K. Patent. However, it has also been found that treatment of an absorbed catalytic metal on a substrate having both nonconductive portions and metallic portions, such as a printed circuit board substrate, with a sulfide solution results in a formation of a sulfide on metal surfaces in contact with the solution of the sulfide precursor solution. Therefore, if the process is used in the manufacture of printed circuit boards, both the catalytic metal and the copper cladding or conductors of the printed circuit board base material are converted to a tenaciously adherent sulfide. If the copper sulfide is not removed prior to electroplating, it may reduce the bond strength between the copper and a subsequently deposited metal over the copper.
An alternative method for direct electroplating of nonconductors is disclosed in U.S. Pat. No. 4,619,741, incorporated herein by reference. In accordance with the procedures of this patent, a nonconductive substrate is coated with a dispersion of carbon black and then dried. The coating is removed from surfaces where plating is undesired and the remaining portions of the substrate are plated using procedures similar to those described in the aforesaid references. There are several problems inherent in this procedure. For example, carbon black is a poor conductor of electricity, and consequently, before forming the carbon black dispersion, in practice, it is believed that the carbon black particles must be treated with an organic ionomer or polymer to enhance conductivity. In addition, during processing, and prior to electroplating, the carbon black dispersion is only poorly adhered to the underlying substrate and has a tendency to flake off of the substrate prior to the plating step. This results in void formation during plating. In addition, because of the poor adherence to the substrate, subsequent to plating, there is a tendency for the metal deposit to separate from the substrate. This can lead to interconnect defects between a metallized hole and an innerlayer in multilayer printed circuit fabrication.
A more recently utilized direct plate process for metallizing the walls of hole-walls employs dispersions of graphite for the formation of a conductive coating. The use of graphite to form conductive coatings on through-hole walls is known and disclosed in U.S. Pat. No. 2,897,409 incorporated herein by reference. Current processes are disclosed, for example, in U.S. Pat. Nos. 4,619,741 and 5,389,270, each incorporated herein by reference. In accordance with the procedures set forth in these patents, a dispersion of carbon black or graphite is passed through the through-holes to form a coating of the dispersion on the hole-walls. The coating is dried to yield a conductive layer of the carbon black or graphite which is sufficiently conductive for electroplating in a conventional manner.
In both the carbon and graphite processes, because of the poor conductivity of these materials relative to a metal, there is a tendency for the metal deposit to propagate from a conductive surface such as copper cladding in the fabrication of printed circuit boards. Therefore, the thickness of a deposit is uneven, thicker in the areas in closest proximity to the metallic layer and thinnest in the area most remote from the metallic surface. In addition, take-off time for initiation of deposition can be relatively slow, again as a consequence of the relatively poor conductivity of the carbonaceous coating.
In co-pending U.S. patent application Ser. No. 08/206,733 filed Mar. 4, 1994, incorporated herein by reference, an alternative to the use of carbon and graphite dispersions is disclosed. In accordance with the procedure described in this application, a dispersion of other conductive particles is used such as particles of metal, conductive metal salts and conductive oxides. As further disclosure within said application, additional components are added to the dispersion such as conductive polymers, binders, surfactants and reducing agents.